In C++, it is possible to customize the code std::set
uses to sort its arguments. By default it uses std::less
, but that can be changed with the Compare
template parameter.
Rust's BTreeSet
uses the Ord
trait to sort the type. I don't know of a way to override this behavior -- it's built into the type constraint of the type stored by the container.
However, it often makes sense to build a list of items that are sorted by some locally-useful metric that nevertheless is not the best way to always compare the items by. Or, suppose I would like to sort items of a use
d type; in this case, it's impossible to implement Ord
myself for the type, even if I want to.
The workaround is of course to build a plain old Vec
of the items and sort
it afterward. But in my opinion, this is not as clean as automatically ordering them on insertion.
Is there a way to use alternative comparators with Rust's container types?
Custom comparators currently do not exist in the Rust standard collections. The idiomatic way to solve the issue is to define a newtype :
struct Wrapper(Wrapped);
You can then define a custom Ord
implementation for Wrapper
with exactly the semantics you want.
Furthermore, since you have a newtype, you can also easily implement other traits to facilitate conversion:
convert::From
can be implemented, giving you convert::Into
for free ops::Deref<Target = Wrapped>
can be implemented, reducing the need for mapping due to auto-deref Note that accessing the wrapped entity is syntactically lightweight as it's just two characters: .0
.
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